A wide variety of mass spectrometer systems have been proposed. For quadrupole mass spectrometers, there are many designs which include two, three or more quadrupole rod sets. Conventionally, each of these has its own function and often the various quadrupoles will be operated at different pressures. Where an individual quadrupole rod set is operated as a mass analyzer, this is usually independent of the function of adjacent rod sets.
Tandem quadrupole systems have been widely used for so called triple quadrupole MS/MS experiments (See for example U.S. Pat. No. 4,234,791, Nov. 18, 1980 "Tandem Quadrupole Mass Spectrometer for Selected ion Fragmentation Studies and Low Energy Collision Induced Dissociation Therefor" by C. G. Enke, R. A. Yost and J. D. Morrison). In these systems a first quadrupole mass analyzer selects an ion of one particular mass to charge ratio (m/e) from a mixture produced in an ion source. These selected ions then collide with a gas in a second quadrupole operated in RF only mode. The collisions transfer translational energy to internal energy of the ions and cause them to fragment. A mass spectrum of the fragment ions is then obtained with a third quadrupole.
U.S. Pat. No. 4,329,582, May 11, 1982 "Tandem Mass Spectrometer with Synchronized Fields" by J. B. French, shows the use of phase locked RF fields to improve the transmission of a triple quadrupole MS/MS system. The RF voltage applied to all three sections or quadrupoles is synchronized or phase locked with a phase shift preferably of zero between the sections. The rods are spaced apart longitudinally by a very short distance not exceeding r.sub.0, the radius of the inscribed circle within the quadrupole rods.
N. V. Konenkov in an article "Coupling the Analyzers of Tandem Quadrupole Mass Filters" published in the Journal of Technical Physics (Russia) 60, 153-157, 1990 (in Russian), and N. V. Konenkov and V. I. Kratenko in the article "Characteristics of a Quadrupole Mass Filter in the Separation Mode of a Few Stability Regions" published in the International Journal of Mass Spectrometry and Ion Physics 108, 115-136, 1991, show operation of a tandem mass filter with the first rod set operated in the stability zone with Mathieu parameters (a,q)=(3,3) and the second rod set operated in the stability zone with Mathieu parameters (a,q)=(0,7.55). Operation in the region near (a,q)=(0,7.55) can be advantageous to mass analyze ions of high kinetic energy or to produce higher resolution than operation in the conventional mode in the first stability region near (a,q)=(0.2,0.7). However, when ions of a particular mass to charge ratio, m/e are transmitted in the (a,q)=(0,7.55) region, ions of 8.4 m/e and higher are simultaneously transmitted because these have stable motion in the first stability region near (a,q)=(0.2,0.7). Thus an additional mass analyzer is required to prevent these higher m/e ions from reaching the second quadrupole. As described, a single RF power supply is used to provide the RF voltage for the two quadrupoles. The quadrupoles are thus phase locked with zero phase shift (FIG. 20 of that paper). Operation of the quadrupoles is otherwise conventional. In particular, operation in the same stability region, operation with a mass shift between the quadrupoles, operation in other stability regions or combinations of other stability regions, or operation with a phase shift other than zero are not described.
N. V. Konenkov in the paper "Coupling Mass Filters of the Tandem Quadrupole Mass Spectrometer" published in the Journal of Technical Physics (Russia) 61, N12, 120-125 (in Russian) describes operation of a triple quadrupole MS/MS system with the mass analyzing quadrupoles operated in the first or third stability regions and the collision cell, an RF only quadrupole, operated in the first region. As in the U.S. Pat. No. 4,329,582, the quadrupoles are phase locked and the optimum phase shift is claimed to be zero. There is a collision cell between the two mass analyzing quadrupoles, and no mass shift or phase shift other than zero between the quadrupoles is described. There is no suggestion that two quadrupoles essentially be operated to effect a single mass analysis.
In the article "Inductively Coupled Plasma Mass Spectrometry with a Quadrupole Operated in the Third Stability region" by Zhaohui Du, Terry Olney, and D. J. Douglas published in The Journal of the American Society for Mass Spectrometry, 8, 1230-1236, December, 1997 it was shown that with operation of the quadrupole in the third stability region the peaks of a mass spectrum can have unusually sharp sides on both the high and low mass sides. This is advantageous because it means a minor peak can be detected without interference beside a major peak of much greater intensity.
What is common in all these earlier proposals is that each quadrupole rod set is essentially operated independently of the others, that is, to perform a function that is independent of functions being carried out in adjacent rod sets. There is no teaching that functions of adjacent rod sets can be combined in any way, to give a single combined function, such as mass selection, which is enhanced as compared to the function available from a single rod set. While there are proposals to connect rod sets electrically, only U.S. Pat. No. 4,329,582 discusses fixing the phase shifts between the rods; even so, this patent teaches a preferred phase shift at zero and it has not been realized that a set non-zero phase shift can give enhanced-sensitivity, at least in some applications.
In the past magnetic analyzers have also been operated in tandem. For example in the article "A Two Stage Magnetic Analyzer for Isotopic Ratio Determination of 10.sup.4 or greater" by F. A. White and T. L. Collins published in Applied Spectroscopy, 8, 169, 1954 it is shown that two magnetic analyzers can be operated in tandem to produce less peak tailing than either analyzer operated alone. R. Schnitzer and M. Anbar in the Proceedings of the 24th Conference on Mass Spectrometry and Allied Topics, p 361, 1976 describe operation of a Wien filter followed by a magnetic analyzer. These and other applications of tandem magnetic analyzers are described in the book "Mass Spectroscopy" by H. E. Duckworth, R. C. Barber and V. S. Venkatasubramanian published by Cambridge University Press, London 1986 on pages 86-90. However in none of these articles is operation with a mass shift between the analyzers described.